The major fluxes of organic carbon associated with physical transport and biological metabolism were compiled, analyzed and compared for the mainstem portion of Chesapeake Bay (USA). In addition, 5 independent methods were used to calculate the annual mean net ecosystem metabolism (NEM = production -respiration) for the integrated Bay. These methods, which employed biogeochemical models, nutrient mass-balances and summat~on of individual organic carbon fluxes, yielded remarkably similar estimates, with a mean NEM of +50 g C m-2 yr.' (i SE = 7.51, which is approximately 8 % of the estimated annual average gross primary production. These calculat~ons suggest a strong cross-sectional pattern in NEM throughout the Bay, wherein net heterotrophic metabolism prevails in the pelagic zones of the rnaln channel, while net autotrophy occurs in the littoral zones which flank the deeper central area. For computational purposes, the estuary was separated ~n t o 3 regions along the land-sea gradient: (1) the oligohaline Upper Bay (1 1 "L of total area); (2) the mesohaline Wd Bay (36% of area); and (3) the polyhallne Lower Bay (53% of area). A distinct regional trend in NEM was observed along this salinity gradient, with net heterotrophy (NEM =-87 g C m-' yr-') in the Upper Bay, balanced metabolism in the Mid Bay and net autotrophy (NEM = +92 g C m-' y r ' ) in the Lower Bay. As a consequence of overall net autotrophy, the ratio of dissolved inorganic nitrogen (DIN) to total organic nitrogen (TON) changed from D1N:TON = 5.1 for riverine inputs to D1N:TON = 0.04 for water exported to the ocean. A striking feature of this organic C mass-balance was the relative dominance of biologically mediated metabolic fluxes compared to physical transport fluxes. The overall ratio of physical TOC inputs (I) to biotic primary production (P) was 0.08 for the whole estuary, but varied dramatically from 2.3 in the Upper Bay to 0.03 in the Mid and Lower Bay regions. Similarly. ecosystem respiration was some 6-fold higher than the sum of all physical carbon sinks. This general negative correspondence between 1:P ratio and NEM, which occurred among Bay regions, was also evident in data available for organic C fluxes in other coastal ecosystems. An inverse relationship between NEM and P, postulated in a previous study, did not apply to Chesapeake Bay, and closer examination of available data revealed the importance of the loading ratio of DIN:TOC as a key control on coastal NEM. It is proposed here that the general global trend of coastal eutrophicatlon will lead to increasing values of NEM in estuaries worldwide. The management implications of this trend are complex, involv~ng both increased potential fisheries harvest and decreased demersal habitat. K E Y WORDS: Net ecosystem metabolism Production . Respiration Organic carbon . Inorganic nutrients Estuaries. Chesapeake Bay INTRODUCTIONRates of organic production in estuaries and other coastal ecosystems are among the highest in the bio-

We examined microbial methylmercury (MeHg) degradation in sediment of the Florida Everglades, Carson River (NV), and San Carlos Creek (CA), three freshwater environments that differ in the extent and type of mercury contamination and sediment biogeochemistry. Degradation rate constant (k deg ) values increased with total mercury (Hg t ) contamination both among and within ecosystems. The highest k deg 's (2.8-5.8 d -1 ) were observed in San Carlos Creek, at acid mine drainage impacted sites immediately downstream of the former New Idria mercury mine, where Hg t ranged from 4.5 to 21.3 ppm (dry wt). A reductive degradation pathway (presumably mer-detoxification) dominated degradation at these sites, as indicated by the nearly exclusive production of 14 CH 4 from 14 C-MeHg, under both aerobic and anaerobic conditions. At the upstream control site, and in the less contaminated ecosystems (e.g. the Everglades), k deg 's were low (e0.2 d -1 ) and oxidative demethylation (OD) dominated degradation, as evident from 14 CO 2 production. k deg increased with microbial CH 4 production, organic content, and reduced sulfur in the Carson River system and increased with decreasing pH in San Carlos Creek. OD associated CO 2 production increased with pore-water SO 4 2in Everglades samples but was not attributable to anaerobic methane oxidation, as has been previously proposed. This ecosystem comparison indicates that severely contaminated sediments tend to have microbial populations that actively degrade MeHg via mer-detoxification, whereas OD occurs in heavily contaminated sediments as well but dominates in those less contaminated.

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Methylmercury (MeHg) degradation was investigated along an eutrophication gradient in the Florida Everglades by quantifying 14 CH 4 and 14 CO 2 production after incubation of anaerobic sediments with [ 14 C]MeHg. Degradation rate constants (k) were consistently e0.1 d -1 and decreased with sediment depth. Higher k values were observed when shorter incubation times and lower MeHg amendment levels were used, and k increased 2-fold as in-situ MeHg concentrations were approached. The average floc layer k was 0.046 ( 0.023 d -1 (n ) 17) for 1-2 day incubations. In-situ degradation rates were estimated to be 0.02-0.5 ng of MeHg (g of dry sediment) -1 d -1 , increasing from eutrophied to pristine areas. Nitrate-respiring bacteria did not demethylate MeHg, and NO 3addition partially inhibited degradation in some cases. MeHg degradation rates were not affected by PO 4 3addition. 14 CO 2 production in all samples indicated that oxidative demethylation (OD) was an important degradation mechanism. OD occurred over 5 orders of magnitude of applied MeHg concentration, with lowest limits [1-18 ng of MeHg (g of dry sediment) -1 ] in the range of in-situ MeHg levels. Sulfate reducers and methanogens were the primary agents of anaerobic OD, although it is suggested that methanogens dominate degradation at in-situ MeHg concentrations. Specific pathways of OD by these two microbial groups are proposed.